A tuning-fork type gyro-sensor utilizing the Coriolis force is widely used as a sensor to detect the rotation of an object. A tuning-fork type gyro-sensor is simple in structure and can be compact, so that it can be used in cameras as a detector for steadying an image and in a car navigation system.
Japanese Patent Laid Open No. 11-37761 discloses four examples of prior art tuning-fork type gyro-sensors. FIG. 4 shows the overall view of a tuning-fork vibration gyro-sensor described in Japanese Patent Laid Open No. 11-37761.
The conventional tuning-fork type gyro-sensor shown in FIG. 4 has an energy confinement type resonator arranged on the arms. This type of tuning-fork type gyro-sensor detects a change in the rotation speed as a change in the output voltage amplitude of the resonator.
In the conventional tuning-fork type gyro-sensor with the structure shown in FIG. 4, a drive electrode 4 (primary electrode) for flexure vibration is formed on a tuning-fork vibrator 3 which includes two arms 1 and a base 2. Electrodes 5 (secondary electrodes) are arranged in an opposing manner on the front and backside surfaces of each of the two arms 1 to make up the energy confinement resonator. A first drive signal is applied to the drive electrode 4 for flexure vibration of the arms 1 while a second drive signal is applied commonly between the secondary electrodes 5a and 5c to output, from each of secondary electrodes 5b and 5c, a signal whose amplitude is modulated according to the flexure vibration.
In this embodiment of the prior art, the output signal of secondary electrodes 5b and 5c is subject to amplitude modification according to the flexure vibration of the arm 1. However, a change in the amplitude is produced between these two output signals of the secondary electrodes 5b and 5c when the Coriolis force acts on the arms 1 during rotation. Therefore, when the differential signal is taken out from the two output signals, the amplitude difference (amplitude beat component) is produced in the differential signal. Synchronous detection of the amplitude beat component with the first drive signal applied to the drive electrode 4 enables generation of a DC voltage proportional to the rotation speed around the Y-axis in FIG. 4.
Another conventional example from Japanese Patent Laid Open No. 11-37761, FIG. 7, detects a change in the rotation speed as a change in the output frequency by using the energy trapped type resonator. The structure is approximately similar to the one shown in FIG. 4, except that two resonators, which include secondary electrodes, function individually as oscillation elements of two independent oscillation circuits, each outputting different oscillation signals from the secondary electrodes.
In the second conventional example, two oscillation signals are subject to frequency modulation through flexure vibration of the arms respectively, with a frequency difference between these two oscillation signals caused by the Coriolis force during rotation. Therefore, by detecting the frequency difference (frequency beat component) between two oscillation signals and through its synchronous detection with the drive signal applied to the primary electrode, a DC voltage proportional to the rotation speed can be generated.
A third conventional example from Japanese Patent Laid Open No. 11-37761, FIG. 8, has a surface acoustic wave element instead of the energy confinement type resonator shown in FIG. 4.
This prior art example uses the amplitude beat component similarly to the first conventional example to detect rotation.
In addition, the type of gyro-sensor that uses the surface acoustic wave element instead of the energy trapped type resonator shown in FIG. 4 is disclosed as a fourth conventional example. This conventional example uses the surface acoustic wave element as a resonant element of the oscillation circuit, and the detection principle is the same as for the above second conventional example, namely, by detecting change in the frequency difference (frequency beat component) of two resonant signals.
As described above, there are various types of vibration gyro-sensors using the Coriolis force that vary in terms of the material and structure of the vibrator, the arrangement of the primary and secondary electrodes, or the rotation detection method.
However, the following problems concern ordinary vibration gyro-sensors using the Coriolis force, such as the above mentioned conventional examples.
Generally, the materials used for a vibrator are piezoelectric ceramic and crystal. Piezoelectric ceramic is easy to prepare and can be used as a vibrator after performing a dielectric polarization process on the prepared ceramic.
By nature, a piezoelectric ceramic develops localized polarization only in locations where a strong electric field is applied externally, so that after forming the primary electrodes and after the application of a strong electric field between the specified primary electrodes, dielectric polarization can develop in the specified location.
Therefore, piezoelectric ceramic is advantageous since the location of the primary electrodes is relatively less restricted.
However, when compared with quartz crystal, piezoelectric ceramic is generally disadvantageous since it has lower detection sensitivity.
On the other hand, a single crystal material like a quartz crystal has a Q value (quality factor) that is generally higher than the Q value of piezoelectric ceramic.
The use of quartz crystal in tuning-fork vibrators offers such advantages as improved detection sensitivity and superior availability of the detection signal in the signal-to-noise ratio.
However, post-treatment such as localized dielectric polarization that is possible with piezoelectric ceramic is physically impossible when using crystal, and it is necessary to prepare the crystal according to the specified crystal axis beforehand and to arrange the primary electrodes according to the specified crystal axis.
Moreover, the method of preparing the crystal is limited because chemical etching in the specific crystal axis direction (Z direction) is almost impossible. Therefore, when using crystal, the position of the primary electrodes is limited when compared to using piezoelectric ceramic material. Furthermore, it is extremely difficult to form the secondary electrodes in a position where the detection sensitivity is optimum in such a manner that these secondary electrodes do not overlap with the primary electrodes.